If I set the volume mixer to use the built in speakers I see Mozilla Firefox there as an application and sound plays just fine from Mozilla. However when I switch sound to my Logitech USB head set no sound. However if I open Google Chrome the Chrome app appears and an app in sound mixer and sound plays just fine. I can switch back and forth between headphones and desktop speakers with chrome with no problems with the sound,.

Images of volume mixer

What do you see in volume mixer? see screenshot